Adds mojom structure for ui::Accelerator and struct traits

ui/base/BUILD.gn

Index: ui/base/BUILD.gn
diff --git a/ui/base/BUILD.gn b/ui/base/BUILD.gn
index 8def626ed9c8d82f971f705711273c824d40d18a..417514cd495c783faadf4a72218fa651cf28ebfe 100644
--- a/ui/base/BUILD.gn
+++ b/ui/base/BUILD.gn
@@ -874,6 +874,8 @@ test("ui_base_unittests") {
 
   if (is_ios) {
     deps += [ "//ui/resources:ui_test_pak_bundle_data" ]
+  } else {
+    deps += [ "//ui/base/accelerators/mojo:unittests" ]

[Tom Sepez, 2017/03/16 21:48:09]

nit: It doesn't sound like //ui/base/accelerators/mojo:unittests" provides
analogous functionality to //ui/resources:ui_test_pak_bundle_data, so maybe it
should have its own if-statement.

[sky, 2017/03/16 22:02:44]

I'm keeping this as I prefer less conditionals. And I'm going to resist trying
to cleanup this file here as there are too many repeated conditionals (line 881
and 901 are the same!)

   }
 
   if (is_win) {